This application claims the benefit of Korean Patent Application No. 2001-29913, filed in Korea on May 30, 2001, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a method of crystallizing amorphous silicon. More particularly, the present invention relates to a sequential lateral solidification (SLS) crystallizing method suitable for forming polycrystalline silicon having uniform grains.
2. Discussion of the Related Art
Due to a rapid development in information technology, display devices have evolved into instruments that can process and display a great deal of information. While cathode ray tubes (CRT) have served as mainstream display devices, to meet current needs flat panel display devices that are small, light weight, and consume low power, such as liquid crystal displays (LCDs), are becoming increasing important.
LCD devices are typically comprised of two substrates and a liquid crystal layer that is interposed between those substrates. LCD devices produce an image by controlling light transmissivity by varying the arrangement of liquid crystal molecules that are arranged by an electric field.
One LCD substrate includes thin film transistors (TFTs) that act as switching devices. Those TFTs are often formed using an amorphous silicon active layer. On reason for this is that amorphous silicon can be formed on a large, low cost substrate such as glass.
LCD devices also include drive integrated circuits (drive ICs) that control the TFTs. Unfortunately, amorphous silicon does not form a suitable active layer for drive ICs, which are usually CMOS (complementary metal-oxidesemiconductor) devices that require crystalline silicon active layers. Because of this, drive ICs are usually connected to a TFT substrate using a TAB (tape automated bonding) system. This adds significant cost to LCD devices.
Because of limitations of amorphous silicon, LCD devices that incorporate polycrystalline TFT active layers are undergoing research and development. Polycrystalline silicon is highly beneficial because it better suited for use in drive IC devices than amorphous silicon. Polycrystalline silicon thus has the advantage that the number of fabrication steps could be reduced because thin film transistors and drive IC could be formed on the same substrate, eliminating the need for TAB bonding. Furthermore, the field effect mobility of polycrystalline silicon is 100 to 200 times greater than that of amorphous silicon. Polycrystalline silicon is also optically and thermally stable.
Polycrystalline silicon can be formed by depositing amorphous silicon on a substrate, such as by plasma enhanced chemical vapor deposition (PECVD) or low-pressure chemical vapor deposition (LPCVD), and then crystallizing that amorphous silicon into polycrystalline silicon. There are a number of different methods of crystallizing amorphous silicon into polycrystalline silicon, including solid crystallization (SPC), metal induced crystallization (MIC), and laser annealing.
In SPC, a buffer layer is formed on a quartz substrate. Then, amorphous silicon is deposited on the buffer layer. The amorphous silicon is then heated at a high temperature, over 600 degrees Celsius, for a relatively long time. The buffer layer prevents impurities from diffusing into the amorphous silicon. The high temperature causes the amorphous silicon to crystallize. However, the SPC method results in irregular grain growth and non-uniform grain size. Therefore, gate insulators grow irregularly on SPC-formed polycrystalline. This decreases the breakdown voltage of the resulting TFTs. Moreover, the electric properties of the TFTs are reduced because of the irregular grain sizes. Additionally, quartz substrates are expensive.
In MIC, a metal deposited on amorphous silicon induces crystallization at a relatively low temperature. This has the advantage that lower cost glass substrates can be used. However, the deposited metals remain in the silicon layer act as detrimental impurities.
In laser annealing, an excimer laser irradiates an amorphous silicon layer on a substrate for several tens to several hundreds of nanoseconds. This causes the amorphous silicon layer to melt. The melted silicon subsequently solidifies into polycrystalline silicon. In the laser annealing method, crystallization can be accomplished at less than 400 degrees Celsius. Unfortunately, crystallization is relatively poor, particularly if the silicon layer is crystallized using a single laser shot. In practice, re-crystallization is usually performed by irradiating the laser beam about 10 times or so to increase the grain size. Therefore, laser annealing suffers from low productivity. Furthermore, laser irradiation can heat the silicon layer to about 1400 degrees Celsius. Because such temperatures would readily oxidize the silicon layer to produce silicon dioxide (SiO2), laser annealing is usually performed under a high vacuum of 10xe2x88x927 to 10xe2x88x926 torr.
Recently, a new method of crystallization, often referred to as sequential lateral solidification (SLS), has become of interest. The SLS method takes advantage of the fact that silicon grains grow laterally from the boundary between liquid silicon and solid phase silicon. The SLS method can increase the size of the silicon grains that grow by controlling the energy intensity of a laser beam and the irradiation range of the laser beam (reference, Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, 956xcx9c957, 1997). This enables TFTs having channel areas of single crystalline silicon.
A conventional SLS method will be described in detail with reference to the attached drawings. FIG. 1 illustrates a conventional SLS apparatus. In FIG. 1, the conventional SLS apparatus includes a light source 1, an attenuator 2, a focusing lens 5, a mask 6, an imaging lens 7, and a translation stage 10, on which a sample 9 having an amorphous silicon layer (element 20 of FIG. 2A) is situated. The SLS apparatus also includes reflective mirrors 3, 4, and 8 to change the direction of the light. The reflective mirrors 3 and 4 are disposed between the attenuator 2 and the focusing lens 5, and the reflective mirror 8 is disposed between the imaging lens 7 and the translation stage 10.
The light source 1 is beneficially a XeCl excimer laser having a wavelength of 308 nm, or a KrF laser having a wavelength of 248 nm. The attenuator 2 controls the energy of the laser beam through the system. The focusing lens 5 and the imaging lens 7 condense the laser beam, while the focusing lens 5 makes the intensity of the laser beam more uniform The mask 6 forms the laser beam into a predetermined shape.
Therefore, the laser beam from the light source 1 is transmitted through the attenuator 2 and is reflected by the reflective mirrors 3 and 4. The laser beam is then condensed by the focusing lens 5, shaped by the mask 6, and passed through the imaging lens 7. Next, the laser beam is reflected by the reflective mirror 8 onto the sample 9. The translation stage 10 then moves the sample 9 and irradiation is repeated.
FIGS. 2A to 2C illustrate a process of crystallizing an amorphous silicon film using the SLS apparatus of FIG. 1. FIG. 2A illustrates an initial step of crystallizing the silicon film wherein a first laser beam irradiation is carried out at a region xe2x80x9cAxe2x80x9d of the amorphous silicon film 20. As stated above, because the grains of silicon grow laterally from the boundary between liquid phase silicon and solid phase silicon, grains 22a and 22b of the region xe2x80x9cAxe2x80x9d grow from both sides of the xe2x80x9cAxe2x80x9d region. Growth of the grains 22a and 22b stops at the line xe2x80x9cIIaxe2x80x9d where the grains 22a and 22b meet.
FIG. 2B illustrate crystallizing the silicon film when a second laser beam irradiation is carried out at a region xe2x80x9cBxe2x80x9d of the amorphous silicon film 20. The region xe2x80x9cBxe2x80x9d includes part of the region xe2x80x9cA.xe2x80x9d The grains 23a and 23b grow from the boundaries of the region xe2x80x9cBxe2x80x9d. In an xe2x80x9cABxe2x80x9d region, where the region xe2x80x9cAxe2x80x9d and the region xe2x80x9cBxe2x80x9d overlap, the grains 22a of FIG. 2A act as crystallization seeds. The growth of the grains 23a and 23b stop at the xe2x80x9cIIbxe2x80x9d line where the grains 23a and 23b meet. The grains 23a are larger than the grains 22a and 22b, which were formed after the first laser beam irradiation.
In FIG. 2C, a third laser beam irradiation is accomplished at a region xe2x80x9cCxe2x80x9d of the amorphous silicon film 20. Grains 24a and 24b of FIG. 2C grow from boundaries of the region xe2x80x9cCxe2x80x9d. The region xe2x80x9cCxe2x80x9d includes part of the region xe2x80x9cB.xe2x80x9d In the region xe2x80x9cBC,xe2x80x9d where the region xe2x80x9cBxe2x80x9d and the xe2x80x9cCxe2x80x9d region overlap, the grains 23a of FIG. 2B act as crystallization seeds. Therefore, the grains 24a of FIG. 2C are much larger than the grains 23a of FIG. 2B.
The whole amorphous silicon film 20 is scanned by repeated laser beam irradiation. Therefore, polycrystalline silicon with large grains is created. Furthermore, crystallization productivity is high because the number of times the same point is irradiated is small.
However, SLS-grown polycrystalline silicon film tends to have different-sized grains and irregular growing directions. Thus, TFTs fabricated from SLS-grown polycrystalline silicon also has properties that depend on the grain-growth.
FIG. 3 shows a current vs. voltage graph of a TFT made with polycrystalline silicon. Here, the x-axis indicates the TFT gate voltage (Vg) and the y-axis indicates the TFT drain current (Id). Also, each line shows a case (solid lines) in which the direction of the channel passed current and the grain-growth are parallel, a case (short dotted lines) which the direction of the channel passed current and the grain-growth direction form an angle of 45 degrees, and a case (long dotted lines) which the direction of the channel passed current and the grain-growth have an angle of 90 degrees. Those lines are established at drain voltages (Vd) of 0.1 V and 10 V.
As shown in FIG. 3, the smaller the angle between the channel direction and the grain-growth direction, the fewer a number of the grain boundaries there are. This improves the current-voltage characteristics. Therefore, if the channel direction of a TFT is parallel with the grain-growth direction, the TFT properties are enhanced. But, if the channel direction of a TFT and the grain-growth direction are at an angle of 90 degrees, the properties of TFT are minimized.
Because SLS-grown polycrystalline silicon has irregular grain-growth directions it is difficult to maximize the properties of TFT formed on SLS-grown polycrystalline silicon. Indeed, it is even difficult to produce TFTs with uniform properties.
Therefore, a technique that improves the grain-growth properties would be beneficial.
Accordingly, the present invention is directed to a method of manufacturing polycrystalline silicon using a mask that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
An advantage of the present invention is that it provides for polycrystalline silicon having large grains.
Another advantage of the present invention is that it provides for a method of manufacturing polycrystalline silicon with more uniform sized-grains.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and in the claims and appended drawings.
To achieve these and other advantages, and in accordance with the purpose of the present invention, as embodied and broadly described, a sequential lateral solidification mask includes a first region having a plurality of first stripes and a plurality of first slits that are located between the first stripes. Additionally, the mask includes a second region having a plurality of second stripes and a plurality of second slits located between the second stripes. The plurality of second stripes is perpendicular to the plurality of first stripes. Additionally, a third region has a plurality of third stripes and a plurality of third slits located between the third stripes. The third stripes and the third slits correspond to the first slits and the first stripes, respectively. A fourth region has a plurality of fourth stripes and a plurality of fourth slits located between the fourth stripes. The fourth stripes and the fourth slits correspond to the second slits and the second stripes, respectively.
In the above mask, the stripes of the first region to the fourth region are made of a light shielding film, and the width of each stripe is smaller than or equal to the width of each slit. Also, the mask can be subsequently arranged in order of the first region to the fourth region, or arranged in order of the first, third, second, and fourth regions.
In another aspect, a method of crystallizing a silicon film uses a mask having a first region having a plurality of first stripes and a plurality of first slits that are located between the first stripes, and a second region having a plurality of second stripes and a plurality of second slits that are located between the second stripes, wherein the plurality of second stripes are perpendicular to the plurality of first stripes. Additionally, the mask used in the method includes a third region having a plurality of third stripes and a plurality of third slits that are located between the third stripes, wherein the third stripes and the third slits correspond to the first slits and the first stripes, respectively. Furthermore, that mask includes a fourth region having a plurality of fourth stripes and a plurality of fourth slits that are located between the fourth stripes, wherein the fourth stripes and the fourth slits correspond to the second slits and the second stripes, respectively. The method includes a step of setting the mask relative to a substrate having an amorphous silicon film, applying a first laser shot to the silicon film through the mask, thereby first portions that correspond to the slits are crystallized, moving the substrate having the crystallized first portions by a quarter width of the mask; and applying a second laser shot to the silicon film. The process continues by moving the substrate by quarter mask widths and irradiating the silicon film.
In the above-mentioned method, the laser beam is irradiated four times at one region of the silicon film. Additionally, the stripes of the first region to the fourth region of the mask are made of a light shielding film, and a width of each stripe of the mask is smaller than or equal to that of each slit. Also, the mask can be arranged subsequently in order of the first region to the fourth region, or arranged in order of the first, third, second, and fourth regions.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.